Retroreflective articles are made in a variety of forms, including sheetings such as are used on traffic signs and license plates, rigid safety reflectors mounted on motor vehicles and bikes, patches and appliques such as are applied to garments and book bags, etc. One major use of retroreflective sheeting is in the field of highway markings and signage to improve the visibility and legibility of informational signs, traffic directions, barriers, etc. to drivers.
One common type of retroreflector employs transparent microspheres, typically with hemispheric reflectors thereon. Illustrative examples of such retroreflectors are disclosed in U.S. Pat. Nos. 3,190,178 (McKenzie), 4,025,159 (McGrath), and 5,066,098 (Kult).
A second common type of retroreflector employs what are referred to as cube-corner elements. Cube-corner retroreflectors typically comprise a sheet having a generally planar front surface and an array of cube-corner elements protruding from the back surface. In use, the retroreflector is arranged with the front surface disposed toward the anticipated location of intended observers. Light incident to the front surface enters the sheet, passes through the body of the sheet to be internally reflected by the faces of the elements so as to exit the front surface in a direction substantially toward the light source, i.e., retroreflection. Illustrative examples of cube-corner type retroreflectors are disclosed in U.S. Pat. Nos. 3,712,706 (Stamm), 4,025,159 (McGrath), 4,202,600 (Burke et al.), 4,243,618 (Van Arnam), 4,576,850 (Martens), 4,588,258 (Hoopman), and 4,775,219 (Appeldorn et al.).
It is known to alter the color of either microsphere-based or cube-cornered retroreflectors by incorporating colorants, e.g., dyes and/or pigments, in overlays which are disposed within the path of retroreflected light or within the retroreflective elements themselves. In addition, incorporation of colorants in the binder layer of microsphere-based retroreflectors has been disclosed.
Retroreflectors are most well known for the capability to retroreflect visible light they exhibit. It would be advantageous in some applications, however, to employ the desirable properties of retroreflectors to selectively retroreflect infrared light without substantially reflecting or retroreflecting visible light. Such retroreflectors would enable the use of signs and markings detectable only by infrared viewing devices located at narrow observation angles relative to an infrared illumination source, while avoiding distracting and/or unwanted detection of the marking by persons in the general vicinity. Such a development would allow inconspicuous marking of and/or signaling by articles for security purposes or identification.
Articles which retroreflect infrared light have been proposed in the past but suffered serious shortcomings. U.S. Pat. No. 3,758,193 (Tung) discloses embedded-lens type retroreflectors comprising overlays containing pigments selected to transmit infrared light and absorb visible light. A disadvantage of such retroreflectors is that they will retroreflect visible light when the overlay is removed. Thus, if the article delaminates or the overlay is scratched, the article may retroreflect visible light undesirably.
U.S. Pat. No. 3,563,771 (Tung) discloses black glass microspheres which are highly transmissive to infrared light and highly absorptive to visible light. A disadvantage of such retroreflectors lies in the production thereof. The manufacture of suitable microspheres for retroreflective products in general is a sophisticated process as discussed in U.S. Pat. No. 4,957,335 (Kuney) for instance. Accordingly, the same facilities used to manufacture IR selective microspheres are typically also used to manufacture microspheres for use in visible light retroreflectors, presenting problems of contamination. Cleaning such facilities of residues from visible light transmissive glass is exceedingly difficult. In one illustrative manufacturing experience, an extensive one-week attempt to remove all visible light transmissive, i.e., uncolored, glass residue from a production facility prior to production of IR selective glass microspheres was unsuccessful. Low contamination levels can impart unacceptable levels of visible light retroreflection to resultant microsphere-based retroreflective articles, depending upon the embodiment. Depending upon the tolerance of acceptable levels of visible light retroreflection, production of IR selective microspheres may require a separate, dedicated production facility, an economically undesirable option.